Light sensitive apparatus



Dec. 5, 1933. A, w U 1,938,426

LIGHT SENSITIVE APPARATUS Filed Sept. 19; 1928 Fig.2.

. Inventor: Albert W. Hu I l His Attorney.

Patented Dec. 5, 1933 UNITED STATES PATENT OFFICE General Electric New York Company, a corporation of Application September 19, 1928 Serial No. 306,945 1 Claims. (Cl. 250-415) The present invention relates to light sensitive apparatus and has for an object the provision of improved circuits for amplifying electrical undulations which represent changes in re- 5 sistance of light-sensitive material in response to light variations. Another object is to provide a circuit arrangement of this sort which may be energized under certain conditions from either an alternating or direct current source. Another feature of the invention contemplates the use of apparatus capable of handling relatively large currents but which nevertheless has a sensitivity of control sufiiciently delicate to respond faithfully to changes in a light source. In view of the fact that I prefer to employ selenium for the light sensitive material, the resistance of which changes readily in response to changes of light having a predominant color component which resides in the long wave length portion of the spectrum, the

invention has particular application to the regulation of a self firing furnace from which a red glow may be derived. Accordingly, a further object of the invention is to provide a sensitive control system for regulating the temperature of a self-firing furnace.

Other objects and features will be apparent as the specification is perused in connection with the accompanying drawing in which Fig. l is a diagrammatical view of the system embodying some of the features of the invention including an electron discharge device and a single light responsive member; Fig. 2 shows the application of the invention to a system employing alternating current and including two light-sensitive bodies combined into a single unit; Fig. 3 representsthe combination of a large power amplifier together with light responsive apparatus, while Fig. 4.- shows in diagram a cascade amplifying system including an electron discharge tube and a power ampliher in combination with the double form of light-responsive device.

Referring to Fig. 1, numeral 1 designates the ordinary form of three-electrode thermionic device comprising a cathode 2, grid or control member 3, and a plate or anode 4. The cathode and plate circuits are energized from a source of current, shown as direct but which may equally well be alternating. In the event the source of energy is of commercial voltage, it is necessary to provide a voltage reducing arrangement for energizing the filament 2, which arrangement may comprise resistances 5 and 6, preferably of equal value, arranged on both sides of the filament and shunted across the line as shown. Between the grid and plate members, there is connected a device 7, the resistance of which changes in response to variations of a light source (not shown). The de- 80 vice 7 conveniently may take the form of a selenium cell or a block of insulation coated in any well-known manner with selenium or other substance, for example, molybdenite, the resistance of which changes in accordance with light variations. A resistance 8 is connected between the negative terminal of the source and the grid member 3. This resistance may have either one of two values depending upon the condition of the device 7 when the circuit is not opcrating and the change in that condition when current through the circuit is initiated by the light source. For example, if it is desired to operate the system from a dark condition of the selenium to an activated condition, the magnitude of resistance 8 should be comparable to that of the device 7 when the latter is in an inactivated or darkened state. On the other hand,if there is normally a light impinging on the member 7 and it is desired to register variations in that light as when comparing light intensities, the magnitude of the resistance 8! should be comparable to that of the device 7 when the latter is activated by the light of normal or standard intensity.

It is apparent that the system operates on the balanced bridge principle wherein current flows from the positive side of the line through the member 7 to charge the grid 3 positively. Current also passes through resistance 8 to charge the grid negatively. When these charges are equal, it is evident that the potential of the grid 3 undergoes substantially no net change from the normal residual negative charge produced by the accumulation of electrons derived from the filament. However, when the magnitude of the resistance 7 is changed by the impingement of light, the balance of the bridge is destroyed and the positive charge transported to the grid preponderates over that of the negative charge causing substantial current to now through the load 9 illustrated in the form of an amme'ter. The ammeter may be useful in comparing two light intensities which successively activate the light-sensitive member 7 although the load may also comprise a sensitive relay for controlling external circuits for any suitablepurpose, for example, switching on or off street or sign-board lights at appropriate times depending upon the amount of daylight 110 present. In the event selenium is employed as the resistance-changing body, the circuit would have application to the control and regulation, by suitably designed relays, of an oil or coal self firing furnace, the glow of which may contain considerable amounts of red light waves to which selenium is particularly responsive. Whatever be the function assigned to the circuit, it is apparent that I have disclosed an extremely sensitive arrangement which responds readily to slight changes in the resistance of device 7, faithfully reproducing the undulations derived from the light source in terms of an electric current flowing throughthe load. In the event that alternating current is utilized for the source of energy, it is apparent that current flow between the anode and cathode may be established only when the anode is positive with respect to the cathode, in which case, the instantaneous polarities of the source of energy under operating conditions, necessarily will be the same as that shown and described.

Alternating current offers some advantages as a source of supply energy in that the voltage reducing arrangement may comprise a transformer instead of resistances 5 and 6. This arrangement has been exemplified in Fig. 2 which in addition, provides a constant resistance light-sensitive member, for example, a selenium cell in a permanently darkened or lighted condition in place of the resistance 8 of Fig. 1. The selenium cells may then be combined on a common block of insulation 10, the two coatings 11 and 12 being in thermal conducting relation with one another so that changes in the ambient temperature aiIects the two coatings equally thereby offering greater accuracy between the changes of space current and the light variations. It is evident that the variable light source should activate only one of the coatings l1 and 12, the inactivated coating being shielded as far as practical from the effects of the variable light. As in the case of Fig. 1, a balance may be obtained in the circuit and a normal negative charge be allowed to accumulate on the grid 3 by collection of electrons slightly in excess of that at which a plate current would be initiated so that when the selenium coatings are activated unequally as when the light falls on coating 11 while coating 12 remains in the dark or inactivated, grid 3 assumes a positive charge with the result that the filament-plate impedance is materially reduced and space current flows through load 13 represented as resistance. The initial balance may be obtained in any suitable manner, for example, by adjusting the position of the filament taps along the secondary of the transformer 14; obviously this adjustment should be such as to produce the maximum degree of sensitivity of the system in response to the light changes which condition usually is obtainable when the bridge is balanced.

Fig. 3 illustrates the application of the invention to a power amplifying tube, as disclosed and claimed in my copending application, which has matured into Patent No. 1,880,092, granted Sept. 2'7, 1932. A device of this sort is characterized by a relatively large plate current of the order of several amperes which effectively may be controlled by an exceedingly small grid energy of the order of a fraction of a micro-watt. Hence, a tube of this character may operate directly a large current relay in the output circuit without the necessity of further amplification. In order to derive a large current-carrying capacity, a cathode of unusual design is employed having considerable surfacial area to emit electrons profusely at reduced operating temperatures; the grid or control member is also of special form. Upon the application of suitable alternating voltage, to each of the cathode, cathode-grid and cathode plate circuits, a discharge is produced which assumes an arc-like character, i. e., has a negative or substantially zero volt-ampere characteristic, the discharge beginning in each alternating current voltage cycle at a time dependent upon the magnitude of the grid bias and continuing until the end of the cycle. Upon formation of the discharge, the grid loses control and the are continues for the remainder of the half cycle during which the anode member'is positive. However, when the alternating current reverses, the discharge stops abruptly and control is restored to the grid at the time the anode starts to become positive again. As shown in the drawing, the arc-discharge device takes the form of an evacuated envelop 15, containing a cylindrical cathode, an anode l7, and a control member 18, enclosing the cathode. The latter is of multicellular design and constructed to provide a plurality of cavities open to the exterior and having a surface which emits electrons profusely at reduced operating temperatures; these cavities conveniently may comprise the space between longitudinally extending metallic vanes 19 arranged equidistantly about and extending between the peripheries of two concentrically mounted cylinders 20 and 21. The cylinders may consist of nickel, or an alloy of tungsten or molybdenum containing thorium oxide. Within the inner cylinder 20 is a heater 22 in the form of a coil or helix, one end of which is connected by welding, riveting, or otherwise, to the thickened end wall 23 of the inner cylinder. The lower end of the heater may be attached by welding or other suitable method, to a stout leading-in conductor 24. In order to prevent the coil heater from striking against the interior metallic surface of the cylinder 20 and thus cause a short circuit, there is provided a lining 25, consisting of a refractory insulating material such as magnesia, alumina or thoria, snugly fitted within the cylinder. The cathode, as a whole, may be supported from the reentrant stem 26 by means of two rods, 27, 2'7, affixed to the outer cylinder 21, one of the rods serving as a leading-in conductor for the heater. The electrostatic control or grid member 18, consists of suitable sheet metal with holes punched therein to a number and size depending upon the amplification characteristic desired, and formed as a cap which sets over the cathode. It is rigidly supported in this position by wires 28, 28 which are attached to a clamp 29 securely mounted on the stem 26. The location of the grid surrounding the cathode and spaced closely adjacent thereto causes the latter to be entirely shielded by the grid from the electrostatic influence of the walls of the envelop. The anode 17 is simply a metallic plate of rugged character supported by a conductor which passes through the upper portion of the envelop. The lower end of the envelop is somewhat enlarged and has coated directly on its surface, preferably the interior, in any suitable manner, two physicallyseparate juxtaposed films 30, 30 of material whose resistance changes in accordance with light variations, for example, selenium. These magmav films are of equal surfacial area and each elec-j trically connected to the clamp 29 and hence, to-

' argon, at apressure of the order of about 5-20 microns or a drop of mercury toevolve vapor sufficient for the initiation of a large currentcarrying discharge under operating conditions.

The device is energized from an alternating current source, preferably of commercial voltage and frequency, the grid and heater circuits being supplied through a transformer secondary 32, the primary 33 of which is connected to the alternating current source; the plate or output circuit is directly across the full line voltage. The output circuit includes a direct current relay 34 of substantial size adapted to actuate contacts which may control any form of mechanism or circuits which it is desired to operate in response to changes of a light source,

35, for example, a valve in an oil pipe line or coal chute leading to a furnace (not shown).

it is apparent that when the various circuits are energized and the selenium films are both in the dark, the grid member 18 derives equal positive and negative charges from the transformer, one. of the charging currents flowing through one of the films and the other passing through the opposite film in the same manner as is explained with reference to Fig. 2. The resistance of each film is so large as to prevent an appreciable amount of current flowing to the control member, in fact, there is only sufilcient current to eifectively charge this member. Under these conditions, it is evident that the average potential of the grid with respect to the average potential of the cathode cylinder 21 is determined by the number of turns between the center point of the secondary 32 and the end of the transformer attached to said cylinder. When the instantaneous voltages of the system are such that the anode 17 becomes positive and the number of turns of winding 32 gives a higher voltage than that of the line, the grid assumes a negative charge and its electrostatic field effcctively prevents a discharge between the anode and cathode members. When the control electrode becomes positive on reversal of the current, the anode is negative and hence no current can flow for well understood reasons. Accordingly, the charge on the grid is eifective in preventing discharge through the device. However, when one of the films, for example, the one on the right as seen in the figure, is activated by the light source 35, indicated generically as an incandescent Amp but which may constitute the glow from a self-firing furnace and the other remains in a darkened or inactivated state, the resistance of the activated film decreases, with the result that a greater charging current fiows through this film than through the other and the equilibrium potential of the grid with respect to the cathode undergoes a change. The grid now assumes a potential corresponding to a position in the secondary intermediate the center tap and the right-hand terminal, i.e. the magnitude of the grid potential approaches that of the cathode. The average valueof the alternating current bias accorded the grid is a measureof the unbalanced selenium film resistances, whichis also a measure of the intensity of light impinging on the activated film. While the position of the filament taps has been shown at the extreme right-hand: end of winding 32, it is obvious that they may be adjusted to accord a proper potential balance to thegrid or rather to apply on the grid a charge of suitable polarity and magnitude in order that the system may become highly sensitized. It is also apparent that this adjustment should be such that when the films 30, 30. are both inactivated, there is substantially no current flow through the relay.

34. As in the case of Fig. 2, the condition of no current through relay 34 may be consonant with the condition of balance in the bridge circuit. The position of the filament taps should be such, that when the reference light changes the resistance of one of the films, the charge on the grid may cause an arc discharge to take place between the cathode and anode members.

As a result of the discharge, a relatively large rectified current flows through the output circuit to energize the relay 34. It will be evident to those skilled in the art that the position of the selenium within the envelop is considered preferable to the exterior coating on a block of insulation shown in Fig. 2 for the reason that the envelop and the enclosed vacuum affords considerable mechanical and moisture protection respectively to the films. The construction in Fig. 3 represents the invention in simplified form and constitutes a compact, rugged and dependable unit. While the position of the selenium films or layers on the interior surface of the envelop represents a very simple construction, it is evident that, if desired, the lightsensitive material may be coated on an insulating, heat conducting member, for example, glass, mounted within the, envelop in any suitable manner. It is also apparent that an electrostatically controlled arc-discharge tube such as described, with the exception of the light-sensitive films, would be admirably adapted as an amplifier in place of the hard vacuum tubes shown in Figs. 1 and 2 when connected up in the well-known manner. However, when an arc discharge device is employed in the system shown in Fig. l, alternating current would be necessary for energization purposes instead of direct current as shown.

In Fig. 4, there is illustrated a cascade amplifying system, including a high. vacuum tube feeding into an arc-discharge device wherein the ultimate load demands an unusual and exceedingly heavy current. The first stage of the system is identical with that described with respect to Fig. 2 and it is obvious that when the balance of the bridge is disturbed suificiently to cause an, appreciable current to flow in the plate circuit of the tube 1 the voltage drop across the resistor 36 is applied between the grid and the cylindrical cathode of the arc-discharge device charging the grid 18 negative to cut off the plate current flowing through the load device 34. If it is desired to actuate or energize the load device during the time the bridge is unbalanced, it is apparent that a coupling may be provided between the tubes of such a nature as to apply a positive charge on the grid during this period, for example, a transformer, may be substituted for the resistance 36. The entire system may be energized from an alternating current source through transformer 37 and the various taps for V the filament of the high vacuum tube may be so chosen to give the greatest overall sensitivity to the system, as is explained with respect to the preceding figures.

It will be obvious that various and diilerent structures and arrangements may be made utilizing the principle and effecting more or less the eflicient results of the invention and that the specific structures shown are illustrative of embodiments of the invention without intention of restricting the scope of the claims to any particular detailed form, arrangement or construction of the device, except as specifically set forth in the claims hereto appended.

' The novel features of the tube shown in Fig. 3 are not claimed in this application but are claimed in my divisional application Serial No. 686,749, filed August 25, 1933 and entitled Light-sensitive apparatus.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a light-responsive system, a thermionic device comprising cathode, grid and plate members, circuits therefor including a light-sensitive device connected between the cathode and grid members, a second light-sensitive device connected between the grid and plate members, said devices consisting of material, the resistance of which changes in response to light variations and being in thermally conducting contact relation but electrically insulated from one another, means fol-energizing said system and a load in the plate circuit.

2. In a furnace temperature regulator, an-

envelop containing an anode, a cathode and a control electrode normally biased to prevent discharge, each of said electrodes being immersed in an attenuated gas, a plurality of light-sensitive members aflixed to the envelop and connected to a source of potential, one of said members serving to charge the grid positive and the other member serving to charge the grid equally negative whereby the charges are neutralized and the normal grid bias remains, and means operable upon activation of one of said light-sensitive members by the glow derived from the furnace for causing a discharge of arclike character to flow between the anode and cathode whereby an external circuit is energized for regulating the furnace temperature.

3. In a furnace temperature regulator, an

but to allow the formation of a discharge when either of said layers are activated by light derived from the furnace, and means responsive to said discharge for regulating the furnace temperature.

4. In combination, a thermionic amplifier including anode, cathode and control elements, a balanced bridge circuit connected between the control element and each 0. the anode and cathode elements, said circuit including a plurality of light-sensitive members, one of said members being connected in the anode portion and another in the cathode portion of the bridge, said members consisting of selenium coated on a common block of insulation material in thermal conducting relation with one another so that changes in the ambient temperature affect the two coatings equally.

5. In combination, a thermionic amplifier of the gaseous type and including anode, cathode and control elements, a bridge circuit connected between the control elements and each of the anode and cathode elements, said circuit including a plurality of light-sensitive members, said members consisting of material, the resistance of which changes in response to light variation, one of said members being connected in the anode portion and another in the cathode portion of the bridge, an alternating currentsource for energizing the amplifier and said members, the voltage of said source being sufiicient to support an arc-like discharge between the cathode and anode elements, said bridge circuit being adjusted to prevent initiation of the arc when the light-sensitive members are inactivated, but to allow initiation of the are when any of said members are activated, said members being secured to a common support of heat-conducting material whereby the lightsensitive members are maintained at the same temperature with respect to one another during changes of ambient temperature and the circuit adjustment of the bridge is maintained.

ALBERT W. HULL. 

